The present invention generally relates to an improvement in the picture quality in a picture (video) coding system, and more particularly to an improvement in a picture distortion, such as a an aliasing distortion or a block distortion, in a picture coding system, particularly, a two-dimensional discrete cosine transform coding system.
In a transform coding system using a two-dimensional discrete cosine transform, an original picture is divided into blocks on a transmitter side. Each of the blocks has a fixed size. A two-dimensional forward discrete cosine transform is carried out for each of the blocks, so that transform coefficients are obtained in a unit of block. Then, the transform coefficients are coded, so that a large amount of information on the original picture is compressed. On a receiver side, the coded transform coefficients are decoded, the results of the decoding are subjected to an inverse discrete cosine transform process, so that the original picture is reproduced.
In this case, generally, an infinite number of terms of transform coefficients is needed to subject the transform coefficients obtained by the forward discrete cosine transform to the inverse discrete cosine transform and completely reproduce the original picture information. However, it is actually impossible to handle the infinite number of terms of transform coefficients. For this reason, generally, the discrete cosine transform is carried out based on an approximate equation which expresses a finite number of transform coefficients equal to the number of pixels which form one block. For example, in a case where one block consists of 8.times.8 pixels, an approximate equation expressing 64 transform coefficients is used.
Thus, it is impossible to completely reproduce a block having a wide distribution of signal levels of pixels, that is, a block having a large amount of high-frequency components, even when such a block is subjected to an inverse discrete cosine transform having a precision equal to that of the forward discrete cosine transform. As a result, there is a possibility that information about a pattern which is not included in the original information is generated. A picture distortion arising from such a cause is called an aliasing distortion or noise. The existence of such an aliasing distortion greatly deteriorates the reproduced picture and provides an impression different from that of the original picture.
From this point of view, in order to configure a high-quality, high-efficient coding system, it is necessary to prevent the occurrence of the aliasing distortion or suppress a visual deterioration of the reproduced picture even when the aliasing distortion occurs.
Similarly, a block distortion must be improved. On the receiver side, the divided blocks are joined together. In a case where neighboring blocks have different characteristics, a boundary between these neighboring blocks visually appears on the reproduced picture. This is called a block distortion or noise. The existence of such a block distortion deteriorates the picture quality. Thus, it is necessary to improve the block distortion.